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Energy and the Environment

Metaschoepite dissolution in sediment column systems – implications for uranium speciation and transport William Bower, Katherine Morris, Francis R. Livens, J. Frederick Willem Mosselmans, Connaugh M Fallon, Adam J Fuller, Louise S. Natrajan, Christopher Boothman, Jonathan R. Lloyd, Satoshi Utsunomiya, Daniel Grolimund, Dario Ferreira Sanchez, Tom Jilbert, Julia E. Parker, Thomas Neill, and Gareth T.W. Law Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b02292 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 22, 2019

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Environmental Science & Technology

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Metaschoepite dissolution in sediment column systems – implications for uranium speciation and transport

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William R. Bower1,2,3, Katherine Morris2, Francis R. Livens1,2, J. Frederick W. Mosselmans4, Connaugh M.

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Fallon1,2,3, Adam J. Fuller1, Louise Natrajan1, Christopher Boothman2, Jonathan R. Lloyd2, Satoshi

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Utsunomiya5, Daniel Grolimund6, Dario Ferreira Sanchez6, Tom Jilbert7, Julia Parker4, Thomas S. Neill2,

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Gareth T. W. Law1,3*

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1. Centre for Radiochemistry Research, School of Chemistry, The University of Manchester, Manchester,

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UK, M13 9PL.

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2. Research Centre for Radwaste Disposal and Williamson Research Centre, School of Earth and

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Environmental Sciences, The University of Manchester, Manchester, UK, M13 9PL.

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3. Radiochemistry Unit, Department of Chemistry, The University of Helsinki, Helsinki, Finland, 00014.

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4. Diamond Light Source, Harwell Science and Innovation Campus, Didcot, UK, OX11 0DE.

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5. Kyushu University, Department of Chemistry, 744 Motooka, Nishi-ku, Fukuoka, Japan, 819-0395.

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6. Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland, 5232.

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7. Ecosystems and Environmental Research Programme, Faculty of Biological and Environmental Sciences,

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The University of Helsinki, Helsinki, Finland, 00014.

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*Corresponding author: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

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Metaschoepite is commonly found in U contaminated environments and metaschoepite-bearing wastes may

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be managed via shallow or deep disposal. Understanding metaschoepite dissolution and tracking the fate of

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any liberated U is thus important. Here, discrete horizons of metaschoepite (UO3●nH2O) particles were

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emplaced in flowing sediment/groundwater columns representative of the UK Sellafield site. The column

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systems either remained oxic or became anoxic due to electron donor additions, and the columns were

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sacrificed after 6- and 12-months for analysis. Solution chemistry, extractions, and bulk and micro-/nano-

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focus X-ray spectroscopies were used to track changes in U distribution and behavior. In the oxic columns, U

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migration was extensive, with UO22+ identified in effluents after 6-months of reaction using fluorescence

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spectroscopy. Unusually, in the electron-donor amended columns, during microbially-mediated sulfate

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reduction, significant amounts of UO2-like colloids (>60% of the added U) were found in the effluents using

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TEM. XAS analysis of the U remaining associated with the reduced sediments confirmed the presence of trace

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U(VI), non-crystalline U(IV), and biogenic UO2, with UO2 becoming more dominant with time. This study

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highlights the potential for U(IV) colloid production from U(VI) solids under reducing conditions and the

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complexity of U biogeochemistry in dynamic systems.


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INTRODUCTION

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Uranium (U) has been released into the environment through mining activities, weapons use, and via

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authorized discharges and accidents at nuclear sites.1-5 It is typically the largest radionuclide by mass in many

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higher activity radioactive wastes that will be managed via geological disposal,6 where U exists in a range of

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chemical forms.7 Uranium is radiotoxic, chemotoxic, long-lived (235U half-life = 703.8 x 106 years, 238U 4.468 x

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109 years), and persists in the subsurface; thus, it poses a significant environmental and human health risk.8,9

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As such, understanding U behavior in the geosphere is essential.

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Uranium is redox-active and its speciation typically controls its solubility and hence mobility. In oxic

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environments, relatively soluble complexes of U(VI) are typically present.5 Under anoxic conditions, lower

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solubility U(IV) species tend to dominate, with reaction end-products including UO2 and poorly-crystalline

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U(IV).10-12 U(V) species have also been documented in environmental studies, but it typically

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disproportionates.13,14 Uranium also readily sorbs to a range of minerals (e.g. Fe/Mn-oxy(hydr)oxides, -

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sulfides, or phyllosilicates)15-21 and cell surfaces,22,23 or it can become incorporated into the lattices of neo-

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forming mineral phases.24,25 Interestingly, incoperation of U(V) into neo-forming Fe oxy(hydr)oxides appears

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to stabilise its redox state.24,26

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Overall, U(VI) reduction, sorption, or incorporation reactions in the geosphere can limit U migration in the

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environment, but to date, U biogeochemistry studies have largely focused on mechanisms controlling U(VI)(aq)

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removal from solution (for example, see the review of Newsome et al., 2015).5 However, U(VI) solids have

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also been released to the environment (often in particulate form), or they can concentrate naturally (e.g.

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from U ores, placer deposits etc.). Our understanding of how these U solid phases behave in the environment,

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and how they contribute to U transport is not well defined.

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Metaschoepite (a U(VI) oxide) is a primary product of depleted uranium munition corrosion in former war

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zones (e.g. Gulf and Balkan Wars) and at military test sites.27-30 It also forms upon oxidation of UO2 in the

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environment,31,32 and UO2 can be formed at U contaminated sites during U(VI)(aq) biostimulation,5 or be

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dispersed in the environment during mining activities or through nuclear accidents.1,18,32,33 Large volumes of


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uranium wastes, that include significant amounts of metaschoepite, may also be managed by either shallow

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or deep disposal in the UK.7,34 Despite this, the environmental behavior of metaschoepite, and its impact on

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U transport in complex, dynamic environmental systems is poorly constrained.

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The objective of this study was therefore to track the stability of metaschoepite and the biogeochemistry of

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any liberated U in complex sediment/groundwater systems, under realistic environmental conditions (i.e.

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flowing groundwater, variable redox, dynamic sediment microbial communities). To do this, sediment

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columns representative of the UK Sellafield Ltd. site were doped with discrete horizons of metaschoepite

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particles. The metaschoepite was then subjected to groundwater flow for periods of up to 12-months; here,

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the columns either remained oxic, or became progressively reducing due to the addition of electron donors.

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Column effluent sampling during the experiments showed that metaschoepite was readily dissolved;

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however, in the electron donor amended system, there was significant production of UO2-like colloids during

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microbially-mediated sulfate reduction. Sacrificial sampling of the sediments after 6- and 12-months of

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reaction and bulk, micro- and nano-focus XAS and XRF analyses was used to track changes in solid U

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speciation over time. Here, in oxic sediments U(VI) binding to Fe-bearing mineral phases was shown to be

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important for longer term U(VI) retention. In the reducing sediments, a mix of non-crystalline U(IV) and

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biogenic UO2 governed U retention, but UO2 appeared to be more prevalent over time. Trace U(VI) was also

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detected in the reducing sediments, despite 10-months of sustained sulfate-reduction. The study highlights

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the potential for U(IV) colloid production from U(VI) solids under anoxic conditions and the complexity of U

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speciation in sediment systems.


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MATERIALS AND METHODS

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Metaschoepite Source

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Synthetic metaschoepite powder was used as a metaschoepite source in sediment column studies. Prior to

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use, the metaschoepite was sieved to 25-50 µm; the “particles” in this size fraction were aggregates of

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micron-scale platelets that formed electrostatically bound clusters. Uranium comprised 77 wt % of the solid,

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yielding an average stoichiometry of UO3•1.3H2O. Further detail on the metaschoepite is provided in the SI

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(Section 1).

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Sediment and Synthetic Groundwater

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Sediment representative of the UK Sellafield Ltd. site subsurface was collected from a well-characterized field

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site in Cumbria35 (SI Section 2). The sediments were sampled from the unsaturated near surface (~5-10 cm

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depth) and were confirmed to be oxic at the point of collection, and at the start of experiments (>97% of the

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0.5 N HCl extractable Fe was present as Fe(III)). Prior to use in experiments, the sediment was sealed and

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stored at 4°C for 90% Fe(II) after 364-

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days). Additionally, aqua regia extractions confirmed that in both electron-donor amended columns the

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sediment-associated Mn concentrations were generally lower than comparable oxic columns (Figure S10C),

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consistent with the observed reductive mobilization of Mn (Figure 1).

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Sediment associated U profiles were also measured from aqua regia lixiviants and highlighted differences in

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gross U retention and migration / U loss under the different groundwater treatments. After 182-days flow

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under oxic conditions, ~70% of the U originally added in the metaschoepite source was recoverable from the

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column sediments. The U had migrated from the source throughout the entire length of the column (U

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concentrations varied between 500-1000 mg kg-1 up to 6 cm beyond the initial metaschoepite source) (Figure

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S10D). However, the U mass balance between the sediment extractions and effluents for this system after

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182-days was poor (~27% of the added U was unaccounted for). This may reflect heterogeneity in U

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distribution in the sediments after metaschoepite dissolution (see Figure 3) relative to the coarse-nature of

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sediment sampling for the aqua-regia extractions. After 364-days under oxic conditions, the solid-phase U

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concentration had then fallen to between ~150-250 mg kg-1 throughout the column (Figure S10D),

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presumably as a result of the continued dissolution and/or mobilization of particle associated/sediment-

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bound U(VI). This meant that ~21% of the originally added U remained extractable from the sediment after

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1-year of oxic groundwater flow. The U mass balance between the sediment and effluents for this time-point


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was better (~106% of the added U accounted for); as such, the majority of the metaschoepite derived U had

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been transported out of the columns.

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In the electron-donor amended columns after 182-days of reaction, the transport of UO2-like colloids out of

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the columns (Figure 1D and Figure 2) meant that the total sediment extractable U was significantly lower

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than its oxic counterpart (only 30% of the U added in the metaschoepite remained in the sediments). This

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decreased to 21% after 364-days. The U mass balance (sediment extractions and effluents) for the two

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electron donor amended columns was 90 and 91%, respectively.

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Bulk X-ray Absorption Spectroscopy

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Select samples were analyzed using bulk U LIII-edge XANES and EXAFS. In the oxic systems, at 182-days, the

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sediments downflow from the initial metaschoepite horizon were sampled. For the bioreduced systems,

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sediments from both within and downflow of the horizon were sampled at 182- and 364-days (see Figures 3

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and 4, SI Figures S11 and S12, Spectra A-E). In order to gain insights into the U oxidation state changes in

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these complex samples, XANES data were analyzed by Iterative Target Factor Analysis (ITFA) (Figure S11).55

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Sediments from the oxic system showed U(VI) speciation as expected, and sediments from the electron-

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donor amended systems showed evidence for partial reduction, with ITFA suggesting between 30-80% U(IV)

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across the samples. Here, the amount of sediment associated U(IV) varied with distance from the

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metaschoepite source, and the overall proportion of U(IV) increased from 182- to 364-days (Figure S11). For

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the oxic system, bulk EXAFS fitting parameters and best-fit results confirmed uranyl-like adsorption to the

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sediment in a sample taken from within the metaschoepite source horizon. Scattering contributions from C

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and Fe shells indicated uranyl complexation at the surface of Fe-oxy(hydr)oxides in the presence of

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carbonate. U-C/Fe distances correlate to expected distances for edge sharing carbonato-complexes at the

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surface of iron oxy(hydr)oxides.56 Uranyl retention by sediments in this manner is well documented, as U(VI)

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has been widely shown to sorb (bidentate edge/corner sharing) to a range of Fe/Mn-oxy(hydr)oxides.21, 56-60

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The dominant Fe-oxides in the sediment used in this study are hematite and goethite, which appear to be

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effective at sorbing U(VI) released from metaschoepite.56-58


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For the electron-donor amended systems, a contribution to the spectra from an axial O shell at ~1.8 Å (uranyl

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coordination) was varied based on ITFA fitting of the XANES (Table S3) and this approach provided good fits.

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In the electron-donor amended column system after 182-days of groundwater flow, a uranyl contribution to

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the bulk EXAFS could still be modelled for samples taken from within the initial metaschoepite source

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horizon, as well as 3-3.5 cm downflow from the source, but its importance was reduced relative to U(IV)-O

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coordination (Figure S12, Table S4; spectra B and C). Both spectra could also be modelled with an improved

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fit that included contributions from Fe and C shells. An oscillation was also visible in the Fourier transform at

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~3.8 Å for the 182-day metaschoepite source horizon sample (spectrum B, Figure S12); this may be indicative

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of a U-U shell from nano-crystalline UO2,10,11,46,47 however, the addition of a U-U shell did not yield a significant

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statistical improvement to the overall model fit for this spectrum.

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After 364-days of reaction in the electron-donor amended column system (Figure S12, Table S4; spectra D

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and E), the inclusion of partial uranyl O coordination was still statistically significant, suggesting that even

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after 10-months under sulfate-reducing conditions, some U(VI) was still present in the sediments. The sample

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within the original metaschoepite source horizon was modelled with a significant U-U backscatter shell, with

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3.2 U atoms at 3.84 ± 0.03 Å, suggesting that nano-crystalline UO2 formation increased with time.e.g.61

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Interestingly, the sample at a greater distance from the original metaschoepite source horizon did not show

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an improved fit with inclusion of a U-U backscatter shell at ~3.8 Å, suggesting no strong evidence for nano-

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crystalline UO2 at distance from the original metaschoepite source horizon.11,12 The formation of biogenic,

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non-crystalline U(IV)10,11 as a U(VI) reductive end-point, separate from that of UO2 formation, is influenced

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by the species present in the experiment. The concentration of organic ligands, phosphate and the availability

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of surfaces of certain Fe(II)-minerals promote formation of non-crystalline U(IV) over UO2 formation.10,12,61-64

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An Fe backscattering shell could also be modelled for all bulk EXAFS spectra collected from the electron-

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donor amended columns except for spectrum D (Table S4).

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Overall, bulk EXAFS fitting for samples from the electron-donor amended column system shows a decrease

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in U(VI) like coordination with time. After 1-year, at the base of the column where sulfate-reducing conditions


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have existed for the longest, UO2 ingrowth dominates; with monomeric or non-crystalline U(IV) present

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further from the initial U and groundwater (and hence sulfate) source.

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High Resolution Autoradiography and µ-focus XRF and XAS

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Data from the oxic column thin-section, sampled after 182-days of groundwater flow, are shown in Figure 3.

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At the 50 µm pixel resolution provided by the autoradiographs, U migration from the metaschoepite source

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along the groundwater flow-path is clear. After 182-days in the oxic column, a concentrated zone of

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radioactivity extends for ~4 cm beyond the metaschoepite source region (Figure 3-II). Beyond this, the activity

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becomes diffuse. µXRF mapping of selected areas within this zone showed a non-uniform distribution of U

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throughout the sample. Many areas showed U enrichment in regions also rich in Fe and Mn (e.g. Figure 3;

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map 1) as well as in the sediment matrix between larger mineral grains (i.e. clay/organic rich areas) (e.g.

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Figure 3; Panel 4). Clays and organic matter commonly sorb U(VI).57,65 The area of highest U retention in the

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thin-section was found along a horizontal, linear feature in the autoradiograph. XRF mapping of this feature

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(Figure 3; maps 2 and 3) indicates it is a root fragment, due to the presence of life-supporting nutrients such

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as Cu and Zn66,67 and consistent with organic material acting as sorption sites for U(VI) in sediments.68,69

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Elemental correlation plots from broad-scale maps (Figure 3-IV) reveal that, of the elements resolvable by

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µXRF mapping, Fe and Mn show the strongest positive correlations with U in these oxic systems. This is also

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reflected in bulk EXAFS modelling where fits show Fe backscatters are possible, suggesting sorption to Fe-

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oxides59 (Table S4). Zinc and Cu also give an indication of possible U uptake onto organic matter, and a

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correlation between U and Sr may be indicative of uranyl association with carbonate or clay minerals.69

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µ-focus XANES measurements from the same thin-section (Figure S11, spectra F-I) showed that U(VI) was

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dominant (>90%) throughout the column. Corresponding µEXAFS measurements could be modelled with

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uranyl O coordination, similar to that described for the bulk-EXAFS measurement from this column system

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(see SI section 9 for the full dataset and fitting parameters). Further, µ-focus spectra F-I (Figures 3 and S13;

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Table S5) could be modelled as uranyl complexes binding to the surfaces of Fe-bearing minerals consistent

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with U(VI) sorption to Fe-oxides.55-59 The fit for spectrum I, collected from the root fragment, was significantly


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improved by the incorporation of a U-C scattering path at 2.91 ±0.02 Å; this may also reflect carbonato-

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complexed U(VI).21,56,70 Evidence for a U-C path is also apparent in the Fourier transforms of spectra F and H,

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but the incorporation of this path was not statistically valid (Table S5).

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In the electron-donor amended column systems, and consistent with bulk geochemical measurements

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(Figures 1 and S10), the autoradiographs (Figure 4) displayed a ‘patchy’ distribution of radioactivity in both

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the 182- and 364-day columns, highlighting discrete regions of U retention following the onset of sulfate-

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reducing conditions. XRF maps revealed that after 182-days of groundwater flow, U was found in discrete

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zones in the sediment matrix (Figure 4, maps 1-4) in a similar manner to that described for the oxic system

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(U correlated with Fe- and Mn-rich phases, as well as biomarker elements like Zn and Cu). Further, more

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diffuse areas of U enrichment were found with distance from the metaschoepite source region, as well as

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additional areas of discrete U enrichment (Figure 4; maps 1-3).


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Figure 3. (I) Color photograph (10 cm vertical) of the oxic column after 182-days. Note the uniformity of orange / brown color along the core’s length, indicative of Fe(III) dominating throughout. (II) Autoradiograph of the resin-embedded, cross sectioned column profile showing radioactivity distribution along the sediment core (‘warmer’ colors denote higher radioactivity). Solid horizontal lines on the autoradiograph mark the zone of initial metaschoepite particle emplacement. Most of the U had migrated from this zone after 182days. Dashed horizontal lines denote the horizon sampled for bulk U LIII-edge XANES and EXAFS (Spectrum A in relevant Figures and Tables). Numbered boxes within the autoradiographs correspond to µ-focus XRF maps in (III). Maps in (III) are either displayed as element-specific color panels (where warmer colors denote relative higher concentrations) or as multi-element RGB maps (where green = U, red = Fe, blue = Mn, and color intensity denotes relative proportions of each element). (IV) Scatter plots denote correlations between major elements (Fe, Mn, Zn, Sr) and U from broad-scale maps over areas that encompass those shown. Positive correlations denote co-location of the elements at differing concentrations. (V) µ-focus (F-I) U LIIIedge EXAFS (k3 weighted Fourier transforms displayed for clarity, full datasets are presented in SI Section 9) from discrete points marked by lettered boxes on the XRF maps. *denotes that the Fourier transform is nonphase corrected. Bracketed regions above the plots show the expected peak positions for O, C and Fe and U scatterers (non-phase corrected).


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Figure 4. For each timepoint (182- and 364-days) from the electron-donor amended column system: (I) Color photographs showing the sediment profiles (10 cm vertical). Note the ingrowth of FeS (blackening) along the flow direction (base to top). (II) Full-length autoradiographs of the resin-embedded, cross sectioned sediments showing radioactivity distribution in the columns (acquisition times varied for clarity, see SI Figure S11 for U concentrations in digests). Solid horizontal lines denote the region of initial metaschoepite source emplacement, dashed horizontal lines denote the horizons from which bulk U LIII-edge XANES and EXAFS data were collected (Spectra B-E in relevant Figures and Tables). Numbered boxes within the autoradiographs correspond to µ-focus XRF maps in (III). Maps in (III) are either displayed as element-specific color panels (where warmer colors denote relative higher concentrations) or as multi-element RGB maps (where green = U, red = Fe, blue = Mn, and color intensity denotes relative proportions of each element). XRF map 6A is a nano-focus map (pixel size 100 nm) of a FIB section extracted perpendicular to white line in XRF map 6 (see Figure S15 for details). Boxes marked J - S, within the XRF maps denote regions from which µ-focus U LIII-edge XAS data were collected, with EXAFS shown in (IV) (k3 weighted Fourier transforms displayed for clarity, full datasets and XANES are presented in SI Section 9). *denotes that the Fourier transform is non-phase corrected. Bracketed regions above the plots show the expected peak positions for O, C and Fe and U scatterers (non-phase corrected). (V) Scatter plots denote correlations between major elements (Fe, Mn, Zn, Cu) and U from broad-scale maps over areas that encompass those shown. Positive correlations denote colocation of the elements at differing concentrations.


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After 364-days, most of the U retained on the sediments was largely present in a discrete horizon ~1 cm

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above the initial metaschoepite source. Here, some U was co-located with Fe-rich mineral grains (Figure 4;

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maps 6 and 7), indicating that U may interact with reactive Fe at mineral surfaces. A proportion of the U was

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also matrix-bound (i.e. not associated with mineral surfaces), possibly being retained by clays and/or organic

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matter along groundwater flow paths (Figure 4, map 7).

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µXANES measurements from the 182-day, electron-donor amended system (Figure S11, spectra J-N) showed

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that U(IV) accounted for 40-60% of the U on the solid phase. µEXAFS also revealed a range of mixed

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U(IV)/U(VI) species (Figure 4 and S13, Table S6). Structures indicative of non-crystalline U(IV) could be

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modelled for several spectra. Only spectrum K, collected from a U hotspot ~7.5 cm from the sediment base,

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could be confidently modelled using a U-U backscatter. Most spectra collected from the 182-day thin-section

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could be modelled with C and/or Fe backscattering shells (Table S6). Combined, the bulk and µEXAFS data

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suggest that monomeric U dominates in this sediment system during the early stages of bioreduction. Again,

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Fe surfaces have also been implicated as important sites for both U(IV) and U(VI) retention.71

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After 364-days in the electron-donor amended system, the residual U in the column was primarily contained

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within the black-colored zones where sulfate reduction had occurred (Figure 4). The proportion of U(IV) in

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this region, as modelled from the µXANES data (Figure S11, spectra O-S), was higher (63-80%) likely due to

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longer periods under reducing conditions. Corresponding µEXAFS data (Figures 3 and S13; Table S7) were

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best modelled with small contributions from uranyl-like O backscatters emphasizing that only residual U(VI)

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remained in this system. Backscattering shells of C and/or Fe and increased contributions from UO2 like U-U

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backscatters could also be modelled. For example, spectra O, P, Q, and S (Figure 4) could all be modelled with

468

a U-U path concurrent with nano-crystalline, biogenic UO2. Evidence for non-crystalline U(IV) in these

469

horizons was scarce; only spectrum R (65% U(IV)) showed no evidence of a U-U scattering contribution. When

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considered alongside the bulk EXAFS data for this column system, it appears there is a preferential growth of

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nano-crystalline UO2 near the base of this system with sustained bioreduction.


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Mechanisms for the preferential formation of ‘monomeric’ U(IV) vs. biogenic UO2 are complex. Studies have

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shown the end-product of microbial U(VI) reduction to be dependent both upon the local microbiology and

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geochemistry. Indeed, systems containing higher ionic strength media (elevated concentrations of chlorides

475

and phosphates) have yielded an increased contribution from non-crystalline U(IV) from the same

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bioreducing cell cultures.10,11,62 Studies have also suggested that UO2 formation is prohibited due to the

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retention of U by cell biomass during reduction.11,64 In phosphate-free systems, Gram-positive bacteria

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preferentially produced non-crystalline U(IV) in direct contrast to Gram-negative bacteria, which produced

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nano-particulate UO2 under the same conditions, the differences proposed to be a result of cell envelope

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architecture.62 The formation of discrete U(IV) phosphate and carbonate phases must also be considered in

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dynamic systems like those presented here, although no evidence is apparent in the µEXAFS, and no P was

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supplied in the groundwater. Significant variations in binding site, local biomass, local porewater chemistry,

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and microbiology are therefore competing factors within discrete micro-environments to favor the formation

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of a specific end-member. Here, the interplay of all these factors on the micro-to-nano scale must be

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considered, however it is difficult to isolate a specific mechanism of non-crystalline U(IV) formation in these

486

dynamic systems. In this case, as observed previously, 72,61 residence time (i.e. ageing of the non-crystalline

487

U(IV) fraction) likely accounts for the increase in nano-crystalline UO2.

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Nano-focus XRF and XANES mapping

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Variations in U speciation were also present at the nanometer scale (Figure S14). The extraction of a wafer

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of U contaminated sediment from the electron-donor amended column sediment (364-day) thin-section,

491

along a Fe-oxide grain boundary (Figure 4, map 6A), permitted nano-XRF and XANES mapping at the newly

492

commissioned Diamond Light Source Hard X-ray Nanoprobe (Figure S14). Discrete U LIII-edge XANES data

493

from this sample have been grouped by subtle differences in the spectra, and the result is a nanometer-scale

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U speciation map. Figure S14 indicates the presence of up to three discrete groups of spectra identified within

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the 10 µm x 5 µm FIB section; the most oxidized (blue and green) are more closely associated with the regions

496

of highest U concentration, along the boundaries of Fe-oxide grains.


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497

Implications

498

This work has shown that an array of competing mechanisms control U migration and fate in complex

499

sediment systems during metaschoepite dissolution. Under oxic conditions, metaschoepite dissolution

500

resulted in significant U(VI)(aq) transport; however, U(VI) reaction with Fe-bearing sediment species retarded

501

groundwater U transport and resulted in long-term (~1-year) U(VI) retention. During metaschoepite

502

dissolution under anoxic conditions, significant amounts of UO2-like colloids were produced. As colloids are

503

known to enhance contaminant transport through the geosphere, this finding may be significant (e.g. for the

504

management of sites contaminated with depleted U penetrators, mining sites, or for the geodisposal of

505

metaschoepite containing wastes). However, further work is required to document the colloid formation

506

mechanism(s) and stability, and this should include defining the role that the solid U source (i.e. chemical

507

composition, surface morphology etc.) plays in controlling / instigating U colloid production, and the relative

508

importance of biotic46,48 vs. abiotic49,52 UO2 colloid formation mechanisms. Under anoxic conditions, micro-

509

to-nano-focus XAS also revealed that U(VI), non-crystalline U(IV), and UO2 were all retained in the sediment

510

after metaschoepite dissolution, and UO2 appeared to be a more important U(IV) product with added time.

511

This may be significant in terms of uranium’s general biogeochemistry, as it is thought that UO2 may be more

512

resistant to oxidative remobilisation, but clearly in this study, this is tensioned against significant U(IV) colloid

513

production. Combined, the results highlight the complexity of U biogeochemistry in dynamic, evolving,

514

environmental systems. This complexity, and how it can be modelled at the macro-scale, needs to be

515

considered for contaminated sites and in nuclear waste disposal.

516 517

ACKNOWLEDGEMENTS

518

We thank University of Manchester staff members Steve Stockley, Barry Gale, Lee Paul, Alistair Bewsher, Paul

519

Lythgoe, Dr Heath Bagshaw, and Dr John Waters for technical assistance. Diamond Light Source and the Swiss

520

Light Source are thanked for beamtimes SP15085, SP17270, SP13559, SP18053-1, and 20170741. NERC and

521

STFC are thanked for funding (NE/M014088/1; NE/L000547/1; ST/N002474/1). NE/L000547/1 was part of

522

the NERC Radioactivity and the Environment (RATE) programme, co-funded by Radioactive Waste


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523

Management Ltd. and the Environment Agency. SU was supported by a Grant-in-Aid for Scientific Research

524

(KAKENHI) from the Japan Society for the Promotion of Science (16K12585, 16H04634, JP26257402) and the

525

Mitsubishi Foundation/Research Grants in the Natural Sciences.

526 527

SUPPORTING INFORMATION

528

Contains additional information on sediment, groundwater, metaschoepite, and microbial community

529

characterization / composition, methods / techniques, pH data, and XAS data.

530 531

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